Transparent Conductive Oxide Layer Research Articles

Fabrication and studies on Si/InP core-shell nanowire based solar cell using etched Si nanowire arrays

We report Si/InP core-shell nanowire radial heterojunction array based solar cell. Silicon nanowire (SiNW) arrays were fabricated by room temperature metal assisted chemical etching method on a p-type Si (100) wafer using silver nanoparticles whereas n-InP layer, as a shell, was deposited onto the SiNW arrays by atmospheric pressure metal organic chemical vapor deposition to obtain core-shell radial heterojunctions. A 100 nm transparent conductive oxide layer was deposited onto top of n-InP layer by sputtering. Transmission electron microscope images confirm the formation of Si/InP core-shell radial nanowire heterostructure. From the studies of reflectance spectroscopy, higher absorption of visible photons has been found. Current-voltage measurements on the radial core-shell nanowire heterojunction based solar cell have been taken under dark and an AM 1.5 solar radiation at room temperature. The device is found to provide a conversion efficiency of 4.39% with an open circuit voltage of 0.56 V and a short circuit current density 14.26 mA/cm2 under AM 1.5 solar radiation. The core-shell radial heterojunction solar cell on nanowire arrays shows great improvement of the performance in comparison with conventional nanowire based solar cells. Our study provides new insights into the Si/InP core-shell nanowire based heterojunction which can have potential applications in fabricating nanoscale optoelectronic devices on Si platform.

Effect of Crystallographic Orientation and Nanoscale Surface Morphology on Poly-Si/SiOx Contacts for Silicon Solar Cells.

High-efficiency crystalline silicon (Si) solar cells require textured surfaces for efficient light trapping. However, passivation of a textured surface to reduce carrier recombination is difficult. Here, we relate the electrical properties of cells fabricated on a KOH-etched, random pyramidal-textured Si surface to the nanostructure of the passivated contact and the textured surface morphology. The effects of both microscopic pyramidal morphology and nanoscale surface roughness on passivated contacts consisting of polycrystalline Si (poly-Si) deposited on top of an ultrathin, 1.5-2.2 nm, SiOx layer are investigated. Using atomic force microscopy, we show a pyramid face, which is predominantly a Si(111) plane to be significantly rougher than a polished Si(111) surface. This roughness results in a nonuniform SiOx layer as determined by transmission electron microscopy of a poly-Si/SiOx contact. Our device measurements also show an overall more resistive and hence a thicker SiOx layer over the pyramidal surface as compared to a polished Si(111) surface, which we relate to increased surfaceroughness. Using electron-beam-induced current measurements of poly-Si/SiOx contacts, we further show that the SiOx layer near the pyramid valleys is preferentially more conducting and hence likely thinner than over pyramid tips, edges, and faces. Hence, both the microscopic pyramidal morphology and nanoscale roughness lead to a nonuniform SiOx layer, thus leading to poor poly-Si/SiOx contact passivation. Finally, we report >21% efficient and ≥80% fill-factor front/back poly-Si/SiOx solar cells on both single-side and double-side textured wafers without the use of transparent conductive oxide layers, and show that the poorer contact passivation on a textured surface is limited to boron-doped poly-Si/SiOx contacts.

Application of Al-Doped (Zn, Mg)O on pure-sulfide Cu(In, Ga)S2 solar cells for enhancement of open-circuit voltage

In this study, interfacial recombination observed by activation energy (Ea) is reduced with an improvement in the built-in potential (Vbi) by raising the conduction band minimum (EC) in Al-doped (Zn, Mg)O (AZMO) layer for pure-sulfide Cu(In, Ga)S2 (CIGS) solar cells. It is observed that the optical band gap in AZMO films can be widened from 3.56 to 3.97 eV with increasing Mg/(Mg + Zn) ratio from 0 to 0.23, suggesting the shift of EC toward the vacuum level. AZMO layers with Mg/(Mg + Zn) ratio of 0–0.23 are applied as transparent conductive oxide (TCO) for the pure-sulfide CIGS solar cells. The open-circuit voltage is clearly enhanced from 0.641 to 0.713 V with increasing Mg/(Mg + Zn) ratio from 0 to 0.09 and then decreased to 0.651 V at Mg/(Mg + Zn) ratio of 0.23 in the AZMO layer. Reverse saturation current density (J0) was minimized to 9.4 × 10−7 A/cm2 at Mg/(Mg + Zn) of 0.09, although J0 was 4.7 × 10−6 A/cm2 in Al-doped ZnO (Mg/(Mg + Zn) of 0). From Mott-Schottky plot, it is observed that Vbi for the pure-sulfide CIGS solar cells gradually enhanced with an increase in Mg/(Mg + Zn) from 0 to 0.23 in the AZMO layer. These results suggest that Vbi improves by controlling EC in the TCO layer, which ultimately reduces the recombination at the hetero interface owing to strengthened electric field.

Microfluidic electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering.

Porous, high-surface-area electrode architectures are described that allow structural characterization of interfacial amorphous thin films with high spatial resolution under device-relevant functional electrochemical conditions using high-energy X-ray (>50 keV) scattering and pair distribution function (PDF) analysis. Porous electrodes were fabricated from glass-capillary array membranes coated with conformal transparent conductive oxide layers, consisting of either a 40 nm-50 nm crystalline indium tin oxide or a 100 nm-150 nm-thick amorphous indium zinc oxide deposited by atomic layer deposition. These porous electrodes solve the problem of insufficient interaction volumes for catalyst thin films in two-dimensional working electrode designs and provide sufficiently low scattering backgrounds to enable high-resolution signal collection from interfacial thin-film catalysts. For example, PDF measurements were readily obtained with 0.2 Å spatial resolution for amorphous cobalt oxide films with thicknesses down to 60 nm when deposited on a porous electrode with 40 µm-diameter pores. This level of resolution resolves the cobaltate domain size and structure, the presence of defect sites assigned to the domain edges, and the changes in fine structure upon redox state change that are relevant to quantitative structure-function modeling. The results suggest the opportunity to leverage the porous, electrode architectures for PDF analysis of nanometre-scale surface-supported molecular catalysts. In addition, a compact 3D-printed electrochemical cell in a three-electrode configuration is described which is designed to allow for simultaneous X-ray transmission and electrolyte flow through the porous working electrode.

Thermally evaporated amorphous InZnO thin film applicable to transparent conducting oxide for solar cells

In recent years, amorphous InZnO (a-IZO) film gets recognition for the possibility as a transparent conducting oxide (TCO) layer with outstanding its optical and electrical properties in photovoltaic devices. Generally, the sputtering technique is normally used to deposit a-IZO film, which suffers from plasma damage exerting a negative influence on device performance. Here, we suggest an alternative to deposit a-IZO film with a thermal evaporation technique using a simply customized system to compensate defects by plasma damage. Thermally grown IZO films by controlling two variables of Zn rate and substrate temperature show the distinguishable characters in terms of transparency, conductivity and crystalline structure with a substantial change of Zn content in In1-xZnxO. Zn incorporation in IZO film is considered as a critical key to determining the crystal structure and IZO film quality. In the thermal evaporation system, phase transition occurs from crystalline to amorphous; the IZO film indicates the superior property in the amorphous phase than crystalline structure. We acquired the optimized IZO film with amorphous phase in the relative concentration of In0.73Zn0.27O grown at 200 °C. To demonstrate the feasibility of a-IZO film as a TCO layer, Cu(In,Ga)Se2 solar cell was practically fabricated, which shows excellent performance. It is attributed to the absence of plasma damage during deposition of the a-IZO film, proved by the decrease of shunt resistance in the solar cell.

Investigation of electrical characteristics of Ag/ZnO/Si sandwich structure

In this study, temperature-dependent current–voltage (I–V), frequency-dependent capacitance–voltage (C–V) and conductance–voltage $$(G/\omega{-}V)$$ measurements are carried out for the electrical characterization of a zinc oxide (ZnO) thin film-based diode. The sandwich structure in the form of Ag/ZnO/Si/Al is investigated at temperatures between 220 and 360 K and in the frequency region of 1 kHz–1 MHz. ZnO thin film layer is deposited on a p-Si wafer substrate as a transparent conductive oxide layer by taking into consideration possible electronic applications with intrinsic attractive material properties. At each temperature step, the I–V curves showed about two orders of magnitude rectifying behavior and, according to the Schottky diode relation, the saturation current, zero-bias barrier height and ideality factor were extracted as a function of the temperature. In the case of non-ideal diode characteristics due to the inhomogeneties in the diode as observed from the characteristics of the calculated parameters, effective barrier height values are evaluated. In addition, based on the existence of the interface layer, density of interface states in the band gap region and parasitic resistances were determined by the capacitance measurements.

Development of nanoparticle copper screen printing pastes for silicon heterojunction solar cells

This paper reports the development of copper screen printing pastes for silicon heterojunction solar cells. Nanoparticle copper paste formulations with a varying amount of copper (percentage by weight) were evaluated in terms of printability, line resistance, and contact formation to Indium-Tin Oxide (ITO) transparent conductive oxide layers. The screen-printed Cu samples were cured under vacuum conditions (<300 ppm O2) at temperatures between 200 °C and 400 °C for 30 min. Scanning electron microscopy was used to investigate Cu nanoparticle sintering at the microstructural level and determine optimal curing conditions for the pastes. The optimized Cu paste formulation yielded consistent finger widths between 53 and 60 μm and finger heights above 20 μm. The average specific contact resistivity of the Cu-ITO contact for the best-performing paste formulation under optimal curing conditions was 0.4 mΩ·cm2. The resistivity of printed Cu lines after curing at 400 °C for 30 min was 27 μΩ·cm. In terms of printability and contact resistance to ITO, the paste formulations developed in this study are suitable for application to silicon heterojunction cells. Steps to further improve the resistivity of the printed Cu lines are discussed. Insights from this study revealed the critical influence of Cu paste composition, rheology, screen printing parameters, and curing conditions on the properties of printed electrodes.

Thermal emissivity of silicon heterojunction solar cells

The aim of this work is to evaluate whether silicon heterojunction solar cells, lacking highly emissive, heavily doped silicon layers, could be better candidates for hybrid photovoltaic thermal collectors than standard aluminium-diffused back contact solar cells. To this end, the near and mid infrared emissivity of full silicon heterojunction solar cells, as well as of its constituent materials – crystalline silicon wafer, indium tin oxide, n-, i- and p-type amorphous silicon – have been assessed by means of ellipsometry and FTIR. The experimental results show that the thermal emissivity of these cells is actually as high as in the more traditional structures, ~80% at 8 μm. Detailed optical modelling combining raytracing and transfer matrix formalism shows that the emissivity in these cells originates in the transparent conductive oxide layers themselves, where the doping is not high enough to result in a reflection that exceeds the increased free carrier absorption. Further modelling suggests that it is possible to obtain lower emissivity solar cells, but that a careful optimization of the transparent conductive layer needs to be done to avoid hindering the photovoltaic performance.

Interdiffusion of Zinc-Stannate Buffer Layer and Cadmium Sulfide Heterojunction and Its Correlation with CdCl2 Treatment

In this paper, a highly transparent and conductive Zinc Stannate (Zn2SnO4) buffer layer is integrated between the transparent conductive oxide (TCO) layer and the Cadmium Sulfide (CdS) window layer to improve the optical properties of (TCO\Zn2SnO4\CdS\CdTe) heterojunctions in CdTe\CdS solar cells. This buffer layer is used to reduce the excess interdiffusion between the window and absorber layers when Ultra-thin CdS window layer, d &lt; 100 nm, is employed, and post CdCl2 treatment is applied at high temperature after deposition of CdTe layer. The fabricated Zinc Stannate (Zn2SnO4) buffer layer was found to have ideal transmission, reflectance and absorption properties which helped in introducing good optical properties in the short wavelengths. It has been found that using the Zn2SnO4 buffer layer enhanced the short circuit current Jsc and the open circuit voltage Voc of the solar cells that used Ultra-thin CdS thickness (80nm) by preventing TCO\CdTe direct contact, and led to improved solar cells’ performance and reproducibility.

Interdiffusion of Zinc-Stannate Buffer Layer and Cadmium Sulfide Heterojunction and Its Correlation with CdCl2 Treatment

In this paper, a highly transparent and conductive Zinc Stannate (Zn2SnO4) buffer layer is integrated between the transparent conductive oxide (TCO) layer and the Cadmium Sulfide (CdS) window layer to improve the optical properties of (TCO\Zn2SnO4\CdS\CdTe) heterojunctions in CdTe\CdS solar cells. This buffer layer is used to reduce the excess interdiffusion between the window and absorber layers when Ultra-thin CdS window layer, d < 100 nm, is employed, and post CdCl2 treatment is applied at high temperature after deposition of CdTe layer. The fabricated Zinc Stannate (Zn2SnO4) buffer layer was found to have ideal transmission, reflectance and absorption properties which helped in introducing good optical properties in the short wavelengths. It has been found that using the Zn2SnO4 buffer layer enhanced the short circuit current Jsc and the open circuit voltage Voc of the solar cells that used Ultra-thin CdS thickness (80nm) by preventing TCO\CdTe direct contact, and led to improved solar cells’ performance and reproducibility.

Characteristics of Zn1−xMgxO:B and its application as transparent conductive oxide layer in Cu(In,Ga)(S,Se)2 solar cells with and without CdS buffer layer

Zn1−xMgxO:B (BZMO) films were deposited by metal organic chemical vapor deposition (MOCVD). Their optical and electrical properties were investigated for suitability as transparent conductive oxide (TCO) of the Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells. It is disclosed that with the increase in the [Mg]/([Mg] + [Zn]) of the BZMO up to 0.12 the optical band-gap energy (Eg) is increased to 3.52 eV, and the resistivity is reduced to about 3.1 × 10−3 Ω·cm owing to the increase in the Hall mobiltity. Moreover, the severe free-carrier absorption in the BZMO film is not observed. The BZMO is consequently appropriate as the TCO layer of the CIGSSe solar cells. The K-treated CIGSSe layers were applied as the absorbers of the solar cells. As a result, the 21.1%-efficient CIGSSe solar cell with a structure of Zn0.88Mg0.12O:B/CdS/CIGSSe/Mo/glass is obtained. In addition, the 20.2%-efficient Cd-free CIGSSe solar with a structure of Zn0.88Mg0.12O:B/Zn0.80Mg0.20O/CIGSSe/Mo/glass, fabricated by all-dry process, is realized with high short-circuit current density (JSC) of 39.7 mA/cm2. The high JSC is attributable to no optical loss in CdS buffer layer, no severe free carrier absorption, and the increased Eg of the BZMO layer.

Comparison study of ZnO-based quaternary TCO materials for photovoltaic application

ZnO is of great interest for large-area optoelectronic devices, e.g., flat panel displays, light-emitting diodes, transparent semiconductors, and transparent conductive oxides (TCOs) etc., owing to its good optical properties. In the present study, quaternary ZnO thin films are deposited on a soda lime glass substrate using Mg and group III elements, such as Al, Ga, and In, to enhance the optical and electrical properties. The structural, optical, and electrical properties of quaternary ZnO TCO materials are investigated for improved thin film solar cell performance. All of the films show a uniform microstructure without any void and crack and have a transmittance over 75% in the visible region. They possess comparable band gap differences. Especially, as for the optical properties, a Mg and Ga co-doped ZnO, MgGaZnO (MGZO) thin film shows a high optical band gap of 3.87 eV with a transmittance of about 90% in the visible region. In addition, the MGZO thin film shows improved electrical properties with the lowest resistivity of 3.97 × 10−4 Ωcm, higher carrier concentration of 1.11 × 1021cm−3, high mobility of 14.07 cm2V−1s−1 and lower sheet resistance of 7.39Ω/sq. Moreover, a wide band gap of 3.87 eV is obtained for MGZO thin films, and the performance of a Cu2ZnSn(S, Se)4 (CZTSSe) photovoltaic device fabricated with a MGZO TCO layer is improved owing to higher band gap and outstanding electronic properties. The improved short circuit current results in a device with a MGZO thin film with a power conversion efficiency of 8.79%.

Improved efficiency of Cu(In,Ga)Se2 mini‐module via high‐mobility In2O3:W,H transparent conducting oxide layer

AbstractPolycrystalline W and H codoped In2O3 (In2O3:W,H) transparent conducting oxide (TCO) layers with high electron mobility were found to enhance both the short‐circuit current density (Jsc) and fill factor (FF) of Cu(In,Ga)Se2 (CIGS) modules with glass/Mo/CIGS/CdS/ZnO/TCO structures. An In2O3:W,H TCO layer was formed by reactive plasma deposition without substrate heating, followed by postannealing at 150°C. A phase transition from amorphous to polycrystalline was confirmed during postannealing, and a layer with micrometer‐scale lateral grain sizes evenly covered the rough ZnO surface generated by the naturally textured CIGS. The crystallized In2O3:W,H film showed high electron mobility (apprixmately 80 cm2 V−1 s−1) at a carrier density of approximately 2 to 3 × 1020 cm−3. Therefore, the In2O3:W,H layer achieved lower sheet resistance and lower free carrier absorption than a ZnO:Al layer, which is commonly used in CIGS modules. The CIGS mini‐module employing the In2O3:W,H layer with a designated area of 3.792 cm2 exhibited a higher efficiency (η) of 20.93% (Voc = 3.081 V [translated value per subcell: 0.770 V], short‐circuit current = 33.21 mA [Jsc = 35.03 mA cm−2], FF = 0.775) with improved Jsc and FF, when compared with a mini‐module employing a ZnO:Al layer. The results indicate that polycrystalline In2O3‐based TCOs with high electron mobility are suitable for CIGS solar cells and modules.

Application of ALD-Al2O3 in CdS/CdTe Thin-Film Solar Cells

The application of thinner cadmium sulfide (CdS) window layer is a feasible approach to improve the performance of cadmium telluride (CdTe) thin film solar cells. However, the reduction of compactness and continuity of thinner CdS always deteriorates the device performance. In this work, transparent Al2O3 films with different thicknesses, deposited by using atomic layer deposition (ALD), were utilized as buffer layers between the front electrode transparent conductive oxide (TCO) and CdS layers to solve this problem, and then, thin-film solar cells with a structure of TCO/Al2O3/CdS/CdTe/BC/Ni were fabricated. The characteristics of the ALD-Al2O3 films were studied by UV–visible transmittance spectrum, Raman spectroscopy, and atomic force microscopy (AFM). The light and dark J–V performances of solar cells were also measured by specific instrumentations. The transmittance measurement conducted on the TCO/Al2O3 films verified that the transmittance of TCO/Al2O3 were comparable to that of single TCO layer, meaning that no extra absorption loss occurred when Al2O3 buffer layers were introduced into cells. Furthermore, due to the advantages of the ALD method, the ALD-Al2O3 buffer layers formed an extremely continuous and uniform coverage on the substrates to effectively fill and block the tiny leakage channels in CdS/CdTe polycrystalline films and improve the characteristics of the interface between TCO and CdS. However, as the thickness of alumina increased, the negative effects of cells were gradually exposed, especially the increase of the series resistance (Rs) and the more serious “roll-over” phenomenon. Finally, the cell conversion efficiency (η) of more than 13.0% accompanied by optimized uniformity performances was successfully achieved corresponding to the 10 nm thick ALD-Al2O3 thin film.

Study the JSC loss of full area SHJ solar cells caused by edge recombination

We have demonstrated that the edge recombination effect exists in full area industrial silicon heterojunction (SHJ) solar cells, which can cause significant short-circuit current density (JSC) loss. The mechanism behind this observation was studied using different SHJ cell structures. We demonstrated further that this JSC loss effect can be suppressed effectively by simply controlling the gap between the edge of the transparent conductive oxide layer and that of the cell. Using this strategy, the average JSC of our state-of-art SHJ solar cells was enhanced by 0.36 mA/cm2, resulting in an average efficiency gain of 0.28% absolute.

Aluminum-Doped Zinc Oxide as Front Electrode for Rear Emitter Silicon Heterojunction Solar Cells with High Efficiency

Transparent conductive oxide (TCO) layers of aluminum-doped zinc oxide (ZnO:Al) were investigated as a potential replacement of indium tin oxide (ITO) for the front contact in silicon heterojunction (SHJ) solar cells in the rear emitter configuration. It was found that ZnO:Al can be tuned to yield cell performance almost at the same level as ITO with a power conversion efficiency of 22.6% and 22.8%, respectively. The main reason for the slight underperformance of ZnO:Al compared to ITO was found to be a higher contact resistivity between this material and the silver grid on the front side. An entirely indium-free SHJ solar cell, replacing the ITO on the rear side by ZnO:Al as well, reached a power conversion efficiency of 22.5%.

Similarity of potential-induced degradation in superstrate-type thin-film CdTe and Si photovoltaic modules

Degradation phenomena under high-voltage stress, referred to as potential-induced degradation (PID) in general, were studied in superstrate-type thin-film photovoltaic (PV) modules, such as CdTe and thin-film Si PV modules. Both deterioration in the PV performance and delamination of the transparent conducting oxide (TCO) layer on the glass substrate were observed by the PID test for both PV modules, although a change in the PV parameters during the PID test with negative voltage application was somewhat different between them. It was also found for both PV modules that recovery from PID is accomplished by positive voltage application and that quick and drastic deterioration in all the PV parameters occurs by the second negative voltage application after recovery. It is suggested that the common origin of PID for superstrate-type thin-film PV modules is damage and delamination of the TCO layer.

Characterization of Cd-Free Zn1- xMg xO:Al/Zn1- xMg xO/Cu(In,Ga)(S,Se)2 Solar Cells Fabricated by an All Dry Process Using Ultraviolet Light Excited Time-Resolved Photoluminescence.

Cd-free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with a structure of glass/Mo/CIGSSe/Zn1- xMg xO (buffer)/Zn1- xMg xO:Al (TCO), fabricated by an all dry process, are characterized using ultraviolet light excited time-resolved photoluminescence (UV-TRPL). The impact of bandgap energy ( Eg) values of buffer and transparent conductive oxide (TCO) layers, denoted by Eg of buffer and TCO, is examined. The Eg values of buffer and TCO layers are kept almost similar and varied from 3.30 to 3.94 eV. In this work, UV-TRPL measurement is performed to examine the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in the solar cell structure. It is revealed that the UV-TRPL carrier lifetimes near the Zn1- xMg xO buffer/CIGSSe interface in Cd-free solar cells are increased upon enhancing the Eg of buffer and TCO from 3.30 to 3.94 eV, thus increasing the open-circuit voltage and fill factor. Additionally, short-circuit current density is enhanced up to about 38 mA/cm2 owing to the highly transparent Zn1- xMg xO/Zn1- xMg xO:Al layers. Ultimately, an 18.5%-efficient Cd-free solar cell with the Eg of buffer and TCO of 3.94 eV, prepared by an all dry process, is fabricated, which has the same level of 18.3% for the reference solar cell (glass/Mo/CIGSSe/CdS/ZnO/ZnO:Al).

Influence of the Thickness of a Layer of Potassium Fluoride Incorporated in the CIGS/CdS Interface on the Macroscopic Electrical Parameters of the Solar Cell

In this work, the heterojunction composed of a n-type ZnO transparent conductive oxide (OTC) layer, a n-type CdS buffer layer and a absorber layer based Cu (In, Ga)Se2 p doped is studied under the influence of a KF layer placed in the CIGS/CdS interface. This study was done by varying the thickness of KF using thin-film simulation software named SCAPS-1D. The presence of KF for a doping of the CIGS absorber of 1016cm-3 improves strongly the electrical parameters that are the Vco, the Jcc the FF, the maximum power and the conversion efficiency of the solar cell Ƞ. However, a decrease of the FF and the Jcc is noticed when the thickness of the KF is greater than 30nm causing a deterioration of the performances of the cell.

Current status of transparent conducting oxide layers with high electron mobility and their application in Cu(In,Ga)Se2 mini-modules

Transparent conducting oxide (TCO) films with high mobility and low carrier density exhibit both high conductivity and high transparency in the visible and near-infrared regions. TCO films that can be fabricated at low process temperatures provide opportunities to improve the performance of solar cells and to develop new optoelectronic applications. This paper describes the current status of ZnO- and In2O3-based TCO layers and their application in Cu(In,Ga)Se2 mini-modules. The use of solid-phase crystallized In2O3:W,H layers instead of conventional ZnO:Al or ZnO:B layers increases both short-circuit current density and fill factor, and a high certified conversion efficiency of 20.93% has been achieved via this approach.